The international criterion for the release of radio-contaminated material to non-regulated markets is a maximum activity of 74 Bq/g, with some countries having set even lower limits of activity. In its unpurified state, contaminated nickel may have an activity upwards of 5000 Bq/g, due to the technetium content alone. The decontamination method described and claimed herein is effective to reduce the beta-activity of such materials to levels at which it can be released to non-regulated markets. It applies equally as well to decontaminating copper, cobalt, zinc, and other metals that can be electrolytically deposited from aqueous solutions.
Many processes for the purification and/or decontamination of nickel are known in the art; electrowinning, or electro-refining, is perhaps the most economically advantageous, and is widely used for the purification of non-radioactive nickel. The following are representive of U.S. patents that describe electrowinning processes for the selective deposition of metal from aqueous solutions: U.S. Pat. Nos. 3,853,725; 3,915,828; 4,011,151; 4,146,438; 4,401,532; 4,481,089; 4,537,666; 4,615,776 and 4,792,385.
Electro-refining using aqueous acid electrolytes is known to be effective for the removal of actinides from contaminated nickel; in such a technique the nickel is deposited selectively on a cathode, with the actinide ions remaining in solution due to their lower electrochemical reduction potential. Conventional electro-refining is however ineffective for reducing technetium concentrations in nickel; technetium is found to co-deposit with nickel at the cathode in a ratio that is the same as, or higher than, that in which it is found in the electrolyte.
In U.S. Pat. No. 5,262,019, Snyder et al address the foregoing problem by providing an electro-refining process with separate electrolytic dissolution and electrowinning steps. Contaminated nickel is first electrolytically dissolved in a sulfuric acid electrolyte, followed by treatment of the filtered nickel-laden electrolyte with an ion exchange resin to remove pertechnetate and other ions; the treated electrolyte is then processed in an electrowinning cell to deposit purified nickel at the cathode.
U.S. Pat. No. 5,217,585, also to Snyder et al, describes an electrorefining process in which the technetium-containing nickel is again electrolytically dissolved in an acid electrolyte. The electrolyte is contacted with activated carbon to absorb pertechnetate ions, after which the solution is filtered and transferred to an electrowinning cell, where the nickel is recovered at the cathode. The contaminated carbon is subsequently incinerated to produce technetium-containing ash, which can be encapsulated for disposal.
A technique in which solvent extraction is combined with electrorefining is described in Snyder et al U.S. Pat. No. 5,156,722. Solvent extraction is used to separate heptavalent technetium from the electrolyte in which radio-contaminated nickel is dissolved, followed by electrowinning to recover the nickel.
The process described in Snyder et al U.S. Pat. No. 5,183,541 (as well as in that of U.S. Pat. No. 5,156,722) employs an electro-refining cell that utilizes a semi-permeable membrane. Technetium is chemically precipitated in the anodic compartment, using a variety of agents to reduce it to its tetravalent state, and is removed by filtration. An hydrochloric acid-based electrolyte is used because it is more amenable than sulfuric acid to the chemical precipitation of technetium.